(1) Field of the Invention
The invention relates to a method of forming polysilicon resistors in the fabrication of integrated circuits, and more particularly, to a method of forming polysilicon resistors with reduced resistance variation in the manufacture of integrated circuits.
(2) Description of the Prior Art
It is common in the art to employ resistors within an integrated circuit which will provide an electrical load which assures proper operation of the circuit under electrical current and voltage parameters to which the circuit was designed. One of the several methods of forming resistors is to use undoped or lightly doped polysilicon, a well-known highly resistive material. An unfortunate characteristic of polysilicon resistors is their ability to absorb hydrogen to which the resistors are exposed during semiconductor processing. The hydrogen is absorbed by carrier trapping states at grain boundaries, rendering the traps electrically inactive. This results in resistance reduction and fluctuation of the resistors. The paper, "Influence of Hydrogen on Electrical Characteristics of Poly-Si Resistor" by Nakabayashi et al, Japanese Journal of Applied Physics, c. 1982, pp. 3734-3738, discusses this topic. The source of hydrogen atoms may be a silicon nitride passivation layer deposited by plasma-enhanced chemical vapor deposition (PECVD); that is, silane (SiH.sub.4)+N.sub.2. In Silicon Processing for the VLSI ERA, Vol. 1, by Wolf and Tauber, p. 222, it is revealed that silicon nitride can serve to block the diffusion of atomic hydrogen. In this context, the blocking of hydrogen diffusion is not desirable.
U.S. Pat. No. 5,500,553 to Ikegami teaches forming a metal cap over polysilicon resistors to equalize the change in resistance caused by hydrogen atoms diffusing into the polysilicon. U.S. Pat. No. 5,530,418 to Hsu et al discloses a three-dimensional structure to protect a resistor from hydrogen diffusion. Dummy polysilicon structures absorb the hydrogen. U.S. Pat. No. 5,656,524 to Eklund et al discloses a nitride/oxide stack partially covering the polysilicon resistor.
U.S. Pat. No. 5,728,615 to Cheng et al states that a conventional solution to the absorption of hydrogen by a polysilicon resistor is to cap the resistor with a nitride layer. This conventional nitride cap is formed by plasma-enhanced chemical vapor deposition (PECVD). PECVD silicon nitride has a lower density and a high induced hydrogen concentration than silicon nitride deposited by low pressure chemical vapor deposition (LPCVD). Cheng et al teach a thermal treatment in N.sub.2 with a H.sub.2 ambient to equalize the hydrogen concentration in a polysilicon resistor. U.S. Pat. Nos. 5,236,857 and 5,465,005 to Eklund et al teach forming an oxide or nitride layer over a polysilicon resistor as an insulator. In this process, the oxide or nitride layer is formed over the polysilicon layer and patterned before the polysilicon layer is patterned. The problem of penetrating hydrogen atoms is not addressed by these patents.